[PATCH] allow the IO scheduler to pass an allocation hint to

[PATCH] allow the IO scheduler to pass an allocation hint to

From: Nick Piggin <piggin@cyberone.com.au>

This patch implements a hint so that AS can tell the request allocator to
allocate a request even if there are none left (the accounting is quite
flexible and easily handles overallocations).

elv_may_queue semantics have changed from "the elevator does _not_ want
another request allocated" to "the elevator _insists_ that another request is
allocated".  I couldn't see any harm ;)

Now in practice, AS will only allow _1_ request over the limit, because as
soon as the request is sent to AS, it stops anticipating.